1. Field of the Invention
The present invention relates to circuitry for terminating and attenuating electrical signals. More particularly, the present invention relates to circuitry for isolating and attenuating incompatible signals present on a telephone network, such as a metering pulse signal.
2. Present State of the Art
The primary function of a telephone network (commonly referred to as the public switched telephone network, or PSTN) is to transmit audio signals. Accordingly, telephone networks have been optimized to carry signals having frequencies and amplitudes that fall within the audio spectrum. However this has limited the ability to utilize the telephone network as a transmission medium for other types of signals falling within different frequency spectrums. Similarly, problems can arise when non-traditional telephone equipment, such as modems, are connected to the telephone.
For a device to connect to the telephone network, it must be able to interpret the various signals that are present on the telephone network. In addition, the device must comply with various standards that are imposed by the telephone network. For instance, in the U.S., to connect to the PSTN, the central switching office places a direct current voltage of approximately −48 volts on the telephone line, and requires that a connecting device, such as a telephone or modem, draw little or no current when the device is not in use. This requirement is satisfied by designing the connecting device to have a large, specified impedance when the device is not in use, referred to as the on hook state. This large impedance effectively creates an open circuit which draws little current.
Another example of a PSTN mandated requirement is that when the device is being used (off-hook), the impedance of the device must closely match the characteristic impedance of the telephone network, typically 600 ohms. This ensures that the signal being transmitted over the telephone network is not reflected due to an impedance mismatch. Thus, a connecting device must be capable of presenting different impedances to the telephone network, depending on its connection state.
The design of a telephone network-connecting device becomes more complex when signals having frequencies and amplitudes that are different from conventional audio signals are present on the telephone network. One such signal is referred to as a “metering pulse signal.” While not present in the U.S. PSTN, in some countries, the telephone network generates metering pulse signals that have a carrier frequency at or above 12 kHz. The metering pulse signal permits entities such as small business to obtain instant billing information so that the cost of a telephone call can be identified. By instantly knowing the cost of a telephone call, the business can charge its customers appropriately.
The metering pulse signal, in addition to having a frequency that is typically higher than voice communications, has peak amplitudes on the order or 20 to 40 volts. As such, the signal can be detrimental to the operation of modems and other connecting devices. For example, the signal can saturate the receive amplifier, corrupt the data, and can physically damage electrical components of the connecting device. In addition, the metering pulse signal, like other signals present on the telephone network, is subject to certain specifications. In particular, the metering pulse signal cannot be attenuated by more than a prescribed amount of about 3 dβ at the input terminals of the connecting device.
One solution to the problem presented by signals such as the metering pulse signal is to use an external podule that contains an attenuation filter. However, this approach is not entirely satisfactory because of the additional cost incurred in order to manufacture, stock and distribute the podules. In addition, the customer is burdened with not only additional equipment that must be connected to the modem, but also with the additional cost associated with purchasing the podules.
Thus, it would be an advancement in the art to provide the ability to limit the detrimental effects of incompatible telephone network signals, such as metering pulse signals, on certain network connecting devices, such as modems. Moreover, it would be an advance in the art to provide a protection scheme that doesn't require extra external equipment, such as an external filtering podule.